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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Greg Stark <gsstark@mit.edu>
Cc: Neil Conway <neilc@samurai.com>
Cc: Jim C. Nasby <decibel@decibel.org>
Cc: Christopher Petrilli <petrilli@gmail.com>
Cc: Ying Lu <ying_lu@cs.concordia.ca>
Cc: pgsql-general@postgresql.org
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] "Hash index" vs. "b-tree index" (PostgreSQL
Date: Tue, 10 May 2005 15:50:05 -0400
Message-ID: <25690.1115754605@sss.pgh.pa.us> (raw)
In-Reply-To: <87wtq7q8zk.fsf@stark.xeocode.com>
References: <427F6853.90308@cs.concordia.ca>
	<427F757A.8000407@samurai.com>
	<59d991c405050908271b55e673@mail.gmail.com>
	<427F8321.5030702@samurai.com>
	<20050509163423.GH35026@decibel.org>
	<427F9211.2090008@samurai.com>
	<20050509165133.GJ35026@decibel.org>
	<427FFCD3.1010803@samurai.com>
	<4407.1115698257@sss.pgh.pa.us>
	<428037A2.4060304@samurai.com>
	<6686.1115700888@sss.pgh.pa.us>
	<878y2nsj7i.fsf@stark.xeocode.com>
	<9871.1115733198@sss.pgh.pa.us>
	<87wtq7q8zk.fsf@stark.xeocode.com>

Greg Stark <gsstark@mit.edu> writes:
>>> What if the hash index stored *only* the hash code? That could be useful for
>>> indexing large datatypes that would otherwise create large indexes.
>> 
>> Hmm, that could be a thought.

> Hm, if you go this route of having hash indexes store tuples ordered by hash
> code and storing the hash code in the index, then it seems hash indexes become
> just a macro for a btree index of HASH(index columns). 

No, not at all, because searching such an index will require a tree
descent, thus negating the one true advantage of hash indexes.  I see
the potential value of sorting by hashcode within an individual page,
but that doesn't mean we should do the same across the whole index.

			regards, tom lane



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